"The only small difference from Sass: for element selectors you need the & prefix. In Sass you could write a { color: red } inside a parent, but native CSS requires & a { color: red }."
It was true for a bit, but fixed within 2-3 releases iirc. You can now freely nav { a { color: red; } }
Anyway, extensions are just signed zip files. You can extract them and view the source. BPC sources are not compressed or obfuscated. The extension is evaluated and signed by Mozilla (otherwise it wouldn't install in release-channel Firefox), if you put any stock in that.